The present invention relates to a reading circuit in an optical disk apparatus (hereinafter referred to as "optical disk drive") to transform an analog signal obtained by optically read-out information from an optical disk into digital data.
An optical disk drive employs an optical disk whose surface is provided with a photo-sensitive recording medium on which information is recorded in the form of a pattern of pits. The optical disk drive irradiates the optical disk with a tiny spot of laser light so as to read the recorded information out of the optical disk by means of a reflected beam therefrom. The intensity of the reflected beam is changed in accordance with the presence or absence of a pit on the surface of the optical disk. The reflected beam is transformed into an analog electrical signal by a photo-transducer. The analog electrical signal (hereinafter referred to as the read-out signal) has a waveform which is in proportion to the intensity of the reflected beam. Then, the read-out signal is converted into a rectangular signal by a reading circuit, the rectangular signal representing digital information.
As is well known in the art, the read-out signal includes an AC component representing the recorded information on the surface of the optical disk and an DC component superimposed on the AC component. Therefore, the DC component should be removed from the read-out signal by the reading circuit when the read-out signal is converted into the rectangular signal.
Such a reading circuit is disclosed in U.S. Pat. No. 4,499,570, issued on Feb. 12, 1985. In accordance with this prior art, a read-out signal is supplied to a threshold-value circuit and compared with a reference signal in order to be converted into a rectangular output signal. The reference signal is generated by means of a detector which measures the average DC level of the rectangular output signal. That is, the detector varies the reference signal level in response to the average DC level of the previous rectangular output signal and the present read-out signal is compared with the variable reference level corresponding to the DC component level in the previous read-out signal. As a result, the rectangular output signal thus obtained substantially represents the AC compoment of the read-out signal in digital form.
In above-described reading circuit, the value of the DC component to be removed is generated in response to the previously read-out signal, and therefore, the DC component that is removed from the presently read-out signal does not have the same value as that of the DC component included in the presently read-out signal. In other words, the detector cannot set the reference signal that is supplied to the threshold-value circuit at a level corresponding to the present DC component of the presently read-out signal. Accordingly, in the case that the DC component of the read out signal is abruptly changed by a large amount, the reference signal level is far different from the actual DC component level of the read-out signal, with the result that the read-out signal is compared with an improper threshold value in order to be converted into a rectangular signal. Thus, mis-reading has often occurred.